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exhaust sizing...

jekbrown

jekbrown

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Right now I have a single 3" exhaust on my K5. Edelbrock headers, high flow cat, FM 40 muffler and a turn down just after that. I've ran this set up for a while, mainly to keep things emissions legal. Well, now I don't care about all that. lol!

I want to go true dual (with crossover pipe), ditch the cat and go with smaller tubing so its easier to fit under my rig. 2.25" mufflers and tubing are cheap. I'd probably run a couple of those flexible braided stainless gimzos just after the h-pipe... then a couple of smallish tube-type (aka cherry bomb) mufflers and then turn-downs before the rear axle.

With a single 3" pipe, I have about 7.07 sq in for air to move through.... with dual 2.25" pipes, I'd have about 7.95 sq in. Is this going to be a problem/big deal or is it "all good"?

thoughts?

j
 
jekbrown said:
those flexible braided stainless gimzos just after the h-pipe...

j
Click to expand...

All sounds good except this... That sh*ts hack imo... Not only does it look cheesy, it leaks often too...

just get some 90's and 45's and weld it up...
 
I just wanted to isolate things in case one (or both!) of the pipes gets bashed on a boulder... if they did leak, the leak would only be about 24" from the end of the exhaust system... is it really that big of a deal?

...and yes, TBI 350.

j
 
I think those things are a good idea. hard mounting an exhaust from front to back isn't such a great idea. Lots of OEMs use flex joints in their exhaust too allow movement between the exhaust and engine.
 
It's always been ghetto hack work in our neck of the woods... :wink1:

But whatever.... I've never seen it on anything factory, that must be new cars...
 
I think you might be confusing the old school spiral flex crap with the newer stainless bellows type flex sections.

I agree, the old stuff is ghetto rigging at it's worst.
The new flex sections are a good idea. Most front drivers come with them. I use one on Patch to keep those 4 squirrels from tearing up the rest of the system & I had one on my turbo diesel Ribbit pick-up (ALL of 75 horses instead of 52!).
 
hmmm, learn something new everyday... thanks... I didn't know there was a new style out...
 
If you can keep the same flow area as a 3", I'd probably see about necking it down to 2".

No one can possibly argue that 3" is ok but something that has the same flow area isn't. (all else being equal)

I've been contemplating this myself, the further back you get, the less diameter you should need. If you are insecure, you could always put 4" tips on. :)
 
2" would work... but might limit my selection with respect to mufflers, the stainless braided flex joint thingies etc. I see them it catalogs down to 2.25" all the time. Hmmm, maybe I need to check out Summit's web site some more. Exhaust prices go down as things get smaller... and things are easier to route also (though that isn't THAT much of a problem with a truggy!)....

j
 
yikes, lotta info there... definitely a useful link though. I'm gonna bookmark that one. :thumb:

what do you guys think of Headman Elite Series headers? Here is the description straight from summit...

Hedman Elite Hedders offer extra heavy-duty construction for hard-use conditions.
Hedman Elite Hedders are made with ultra heavy-duty, 14-gauge steel tubing. They feature a bulletproof 3/8 in. flange and a thermal coating for intense heat conditions. Hedman Elite Hedders are designed to take the pounding and vibration of off-road conditions, as well as extreme hard use, the heat buildup found in motor homes during towing, and performance situations. Hedman Elite Hedders can take the abuse and keep on going when other brands fail.
Click to expand...

3/8" is a good beefy flange... and 14ga is decent thickness for tubing. They also have a "thermal coating" which may help fend off the rust monster. :dunno: Anyone running these? I know I've heard not-so-good reviews of the standard (ie cheap) Hedman headers...

j
 
I've got them. You know our climate/lack of salt use, and the underside of them is pretty darn rusty. WAG that they've got 5 more years before they wear through, that would be at least 7. I screwed up and installed them before the engine was completely broken in, and I discolored the coating because of it, but I don't believe that is the reason they are rusty down low...I don't think they got hot enough to hurt the coating down there. In the engine bay they have little or no rust, just the portion that faces the ground.

They hang down from the (stock) framerail quite a bit, and had a BIT of interference with the shackles, but shackle clearance can be fixed. Hanging down is not so easy, but if designing a rig, you might be able to work around that, or not care?

The ball/socket outlets are awesome though, and the overall tube/cylinder head flange thickness seems to be fine, no leaks after a couple of years using stock *manifold* gaskets.
 
The "flow area" really is not the only thing to compare between a single larger pipe and dual smaller pipes. The dual pipes will always have more surface area as compared to a single with similar flow areas, and therefore create more "friction" (boundary layer) and thus restrict flow some. For comparison, dual 2" pipes have slightly less flow area than a single 3" pipe, but around 30% more surface area.......all meaning that the dual 2" pipes will flow a noticably amount less.

Now with that said, I think that dual 2" would probably work okay but not be optimal on a stock 350 TBI. Lower rpms probably would be fine, but you might be able to notice it getting a little choked at higher rpm levels.
 
hadn't thought of the surface area / friction aspect of it. Good call. Maybe 2.25" would work best then. :thinking: I guess I'm gonna have to read that exhaust tech link some more... I'm trying to figure out where the exhaust should go from 3" (collector size) down to the 2.25". Immediately after the collector? After the braided flexhose thingy? after the muffler (a waste of time at that point, there will only be a foot of pipe after that!)????

Dorian, you got a pic of your headers? mainly interested in how far they hang down below the frame and where the collector ends up relative to the shackle hanger. I am going to 56" springs and the hangers will be just about 1" behind the stock position... so long as they stop before that, all should be a-ok.

Bummer that yours have rusted on the bottom... any idea why? rocks chipping the coating or...? Guess its a good thing that tubing is 14 ga. :shocked:

Also, I looked on Hedman's site and they have half a dozen (more?) headers that will fit trucks... you remember which part # you used?

j
 
1569adapter.JPG


I can get better pics if you like. There are some in my gallery here, but the motor wasn't bolted in apparently when I took those, the headers are obviously angled down too far.

I had more, better pics, but I lost my hard drive. I can take more.

69890 are the ones I used.
 
damn, those suckers dooo hang down quite a bit don't they? wonder if all the different ones that Hedman has for 350s go down that far or if they have some that are a lil bit shorter? :thinking:

If ya get some time, I'd like two specific pics.

1) from the side of the rig, at frame level... (or just a measurement w/ exactly how much they hang below frame level)

2) same position, but with the cam lower to the ground... hopefully allowing the shackle mount to be seen. My shackle hanger will be 1" further back than stock... I'd like to know if they are going to cause any kind of issue. :)

j
 
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